Method of casting inoculated metals

ABSTRACT

This invention teaches a method of controlling the grain or solidification structure in castings which have been prepared from inoculated metals. The inoculated molten metal is subjected to inertial effects during solidification comprised of oscillation and steady state rotation. The invention makes it possible to selectively produce in such castings predetermined zones of equiaxed or columnar grain structures.

United States Patent 1191 Bolling et al.

METHOD OF CASTING INOCULATED METALS Inventors: Gustaf- F. Bolling; Gerald S. Cole,

both of Dearborn, Mich.

Assignee: Ford Motor Company, Dearborn,

Mich.

Filed: Feb. 1, 1973 App]. No.: 328,800

Related US. Application Data Continuation-impart of Ser. No. 833,446, June 16, 1969, abandoned.

US. Cl 75/135, 75/138, 75/166 R, 75/168 R, 164/57, 164/114 Int. Cl B22d 1/00, B22d 21/00 Field of Search 164/114-1 18, 164/55,57;75/135,138, 166 R, 168 R References Cited UNITED STATES PATENTS 10/1912 Crawford 164/115 Oct. 29, 1974 2,778,079 1/1957 Carney et al 164/122 ux 2,973,564 3/1961 Dixon et al. 3,415,307 12/1968 Schuh et al. 164/114 Primary Examiner-Robert D. Baldwin Attorney, Agent, or Firm-Keith L. Zerschling; Joseph W. Malleck [57] ABSTRACT This invention teaches a method of controlling the grain or solidification structure in castings which have been prepared from inoculated metals. The inoculated molten metal is subjected to inertial effects during solidification comprised of oscillation and steady state rotation. The invention makes it possible to selectively produce in such castings predetermined zones of equiaxed or columnar grain structures.

16 Claims, 6 Drawing Figures PATENTEIIIIIITZSISM Bi-844776 sum 20F 3 FIG. 4

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CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-impart of our copending application Ser. No. 833,446, filed June 16, 1969, now abandoned, and having the same title as the present invention. This invention should further be considered in view of the teaching of the companion application, U.S.S.N. 760,126, filed Sept. 13, 1968 and now issued as U.S. Pat. No. 3,614,976, by the inventors of the current application and assigned to the same assignee.

BACKGROUND OF THE INVENTION U.S. Pat. No. 3,614,976 teaches the control of grain or solidification structure particularly in castings which are approximate objects of revolution by either slow rotation or very mild angular acceleration about an axis preferably coincidant with the axis of the casting. [For purpose of definition throughout this application, the use of the term acceleration shall comprehend also deceleration] The maximum speed of rotation treated in both the patented invention and the instant application is only of the order of about 60 revolutions per minute for an ingot diameter of about 6 inches. This eliminates all velocity gradients arising from internal buoyancy forces in the fluid. This order of magnitude should be understood to be distinct from the much higher magnitude of rotation employed in conventional centrifugal castings.

In the earlier U.S. Pat. No. 3,614,976, there is described in detail the casting of an uninoculated INCO 713 turbine wheel. The only inoculation taught by the earlier patent is a minor inoculant coating of the mold as opposed to the present invention in which the inoculant is incorporated directly into the molten metal for solidification thereby permitting control of solidification characteristics.

The prior art has not appreciated the unusual selectivity and control of diverse solidification structures that can be obtained within the same casting. The art has known of seeding or inoculation for purposes of grain refinement, but such art has been limited solely to uniformly affecting the entire casting with no intention of providing selective control of the particular solidification structure. The art also has appreciated the use of rotation for affecting mixing of the molten metal during casting. A convenient mechanism which has been employed is the rotation of the casting mold table, the mold being subjected to oscillation to achieve some degree of acceleration and deceleration. However, the resulting solidification structure has always been equiaxed or some relationship thereto.

SUMMARY OF THE INVENTION This invention concerns a discovery that the addition of an inoculant to the molten metal of a casting, which is to be subjected to slow rotation or oscillation during cooling, directly reverses the results obtained with uninoculated castings. In the uninoculated castings, steadystate rotation produces a columnar structure while angular acceleration or oscillation produces an equiaxed structure tending to be fine-grained. In dealing with homogeneously inoculated castings, the direct opposite effect occurs with angular acceleration producing a columnar structure and steady-state rotation producing an equiaxed structure.

SUMMARY OF THE DRAWINGS FIGS. l-6 are metallographic studies which depict the solidification structure of various test examples for illustrating the zoning effect and control of grain structure.

DETAILED DESCRIPTION Turning now to FIG. 1, the casting depicted by the metallographic study was taken from an ingot consisting essentially of pure aluminum alloyed with 6.4 percent silver to which was added 1 percent of aluminum oxide (A1 0 The silver content arises from the convenient mechanism of adding silver oxide to precipitate highly active A1 0 The molten metal preparation was permitted to cool and yield a structure which contained the usual outer zone of columnar grain structure with the conventional inner core of equiaxed material due primarily to the natural convection current set up during the final stages of cooling. Apart from the steps of the present invention, such zones cannot be varied and therefore demonstrate the lack of selectivity by the use of aluminum oxide as a grain refining inoculant.

FIG. 2 depicts a cross section of a similar argentiferous aluminum ingot containing 6.4 percent silver and containing no grain refiner or inoculant. This figure has been presented for a standard of comparison, it shows the structure obtained by the same aluminum with 6.4 percent silver composition and which was simply cooled. The very large grain columnar structure is evident throughout.

FIG. 3 depicts the ingot structure obtained when silver oxide is added to essentially pure molten aluminum so that the final composition will render 1.28 percent silver and 0.2 percent freshly precipitated aluminum oxide. The silver content was obtained by the thermite reaction between the silver oxide and aluminum to yield silver and freshly precipitated and highly active aluminum oxide. This particular casting was poured and for the first 10 seconds after pouring subjected to oscillations at 150 rpm. For the next 15 seconds (that is between 10 and 25 seconds after pouring) it was rotated at rpm. For the next 35 seconds (that is between 25 and 60 seconds after rotation) the .molten metal preparation was again oscillated at rpm. The study of FIG. 3 shows the deep band or layer of columnar metal adjacent the mold section, produced by the original oscillations. The columnar band yields to a layer of equiaxed small grains produced by the 15 second rotation, and then finally there is a further columnar structure produced by the final oscillation. The central portion of the ingot tends to be a fine-grained equiaxed structure because of the inability of either rotation or oscillation to impart acceleration to the centrally located metal.

To show the range of inoculants that can be used with aluminum based alloy, FIG. 4 shows an ingot cast under conditions similar to FIG. 3 except that a commercial aluminum grain-refining agent was added in which the active ingredients were boron and titanium. The inoculant was added in an amount to obtain respectively 0.002 percent by weight in the ingot of each titanium and boron. Again the molten metal preparation was subjected to the same oscillation-uniform rotationoscillation sequence as for FIG. 3. Since FIG. 4 is taken at a slightly higher magnification, the various zones can be more clearly observed. The zones corresponding to the respective inertial effect periods, are essentially equiaxed, columnar, and equiaxed again, proceeding from the outer rim toward the inner core of the ingot.

To more dramatically depict the scope of this invention as applied to other rather diverse metal castings and therefore proving the feasibility of the present invention throughout the metal range known to the art for inoculation (such as depicted in part in US. Pat. No. 2,778,079) two-other diverse systems are presented. A lead-based metal casting system is illustrated firstly in FIG. 5. The casting was provided with a molten metal preparation of lead and to that was added an inoculant in the form of copper in an amount to obtain a final composition having .25 percent by weight copper and the remainder lead. In this type of an alloy system, exact control of temperature is essential for obtaining a proper cast structure which is reproducible.

The molten metal preparation was stirred and cast at 100C superheat. If the temperature control is not carefully adhered to, copper-rich constituents will rise to the surface of the melting crucible prior to pouring. The molten preparation was then cast into graphite molds which were water cooled.

FIG. 5 shows metallographic study of one-half of the ingot. The molten metal preparation of lead containing the inoculant was subjected to a series of oscillationrotation-oscillation effects as labeled in FIG. 5. This is a useful method for examining the selectivity of the solidification structure. The figure shows how coarsegrained columnar structure dominates oscillation sequences and how fine-grained equiaxed structure dominates the uniform rotation sequences. From this example and others, it has been determined that oscillation or angular acceleration must be used and be of a sufficient duration so that a new laminar boundary layer can be created in order to obtain the appropriate solidification structure control. If no inoculant had been present for FIG. 5, but if the mold had been alloyed, the reverse solidification structures would have been obtained.

In FIG. 6, a magnesium system was employed using zirconium as the inoculant, the inoculant being denser than the matrix in opposition to FIG. 5. Stirring the molten metal preparation prior to casting was essential to retain a uniform ingot composition with zirconium constituting 0.6 percent by weight and the remainder magnesium. For this system it was found desirable to air cool the graphite molds during solidification. FIG. 6 shows the columnar-equiaxed-columnar development resulting from sequential inertial effects during the solidification bands labeled respectively osc.-rot.- osc. It is theorized that the tiny nucleated grains, produced by the inoculant, are swept away from the freezing front by the action of acceleration so that columnar grains can progress uninhibited, whereas during uniform rotation these nucleated grains remain in position to block or prevent columnar growth. There is at all times an interaction with heat flow, particularly near the mold wall, where nucleation is inhibited because of high heat flow and high temperature gradients. In the ingot center, wheretemperature gradients are low, columnar growth is difficult to obtain.

Another inoculant that can be used with the lead system is calcium. Other inoculants that can be used with the magnesium system include: aluminum, zinc, calcium, silicon and rare earth elements.

We claim:

1. A method of selectively controlling the solidification structure of a metal object, comprising:

a. adding an inoculant to a molten metal preparation, the inoculant being comprised of material effective to form a group of atoms about which the molten metal nucleates during solidification,

b. subjecting the molten metal during solidification to at least one sequence comprising each of the following inertial effects in either order: a plurality of angular accelerations each at least sufficient to create a new laminar boundary layer in order to produce a columnar solidification structure in that portion of the object solidified during said accelerations, and a substantially uniform rotation predominantly devoid of velocity gradients and angular acceleration to produce an equiaxed solidification structure in that portio of the object solidified during said uniform rotation.

2. The method as in claim 1, in which said molten metal preparation is comprised of aluminum.

3. The method as in claim 2, in which said inoculant is comprised of A1 0 4. The method as in claim 2, in which said inoculant is comprised of at least one of boron and titanium.

5. The method as in claim 4, in which said inoculant is added in amounts such that the final metallic object contains between 0002-0003 percent of either titanium or boron.

6. The method as in claim 1, in which said molten metal preparation comprises lead.

7. The method as in claim 2, in which the inoculant is selected from the group consisting of silver oxide, aluminum oxide, boron and titanium.

8. The method as in claim 6, in which the inoculant is added in an amount sufficient to constitute no greater than 0.30 copper percentage by weight in the metal object.

9. The method as in claim 1, in which the molten metal preparation comprises magnesium.

10. The method as in claim 9, in which the inoculant comprises zirconium.

11. The method as in claim 10, in which zirconium is added in an amount sufficient to constitute no greater than 0.65 percent by weight of the final metal object.

12. The method as in claim 9, in which said inoculant is selected from the group consisting of aluminum, zirconium, calcium, zinc, silicon and rare earth metals.

13. The method as in claim 6, in which said inoculant is selected from the group consisting of copper and calcium.

'14. The process of controlling the grain structure of a solidifying metal object comprising adding to the molten metal an inoculant which would normally produce a fine grained structure in the metal object on solidification, subjecting the metal during solidification to a plurality of mild angular accelerations during the time solidification is taking place in that portion of the object in which a columnar structure is desired, and subjecting the metal to centrifugal force generated by slow rotation during the time solidification is taking place in that portion of the object in which an equiaxed structure is desired.

3,844,776 S 6 15. The process recited in claim 14 in which the 16. The process recited in claim 14 in which the sometal object is essentially an object of revolution and lidifying metal object is subjected to a plurality of mild the angular accelerations and slow rotation have an angular accelerations immediately after casting so that axis which is essentially the axis of revolution of the obthe outer layer of metal is columnar in structure. ject 

1. A METHOD OF SELECTIVELY CONTROLLING THE SOLIDIFICATION STRUCTURE OF A METAL OBJECT, COMPRISING: A. ADDING AN INOCULANT TO A MOLTEN METAL PREPARATION, THE INOCULANT BEING COMPRISED OF METAL EFFECTIVE TO FORM A GROUP OF ATOMS ABOUT WHICH THE MOLTEN METAL NUCLEATES DURING SOLIDIFICATION, B. SUBJECTING THE MOLTEN METAL DURING SOLIDIFICATION TO AT LEAST ONE SEQUENCE COMPRISING EACH OF THE FOLLOWING INERTIAL EFFECTS IN EITHER ORDER: A PLURALITY OF ANGULAR ACCEL ERATIONS EACH AT LEAST SUFFICIENT TO A NEW LAMINAR BOUNDARY LAYER IN ORDER TO PRODUCE A COLUMNAR SOLIDIFICATION STRUCTURE IN THAT PROTION OF THE OBJECT SOLIDIFIED DURING SAID ACCELERATIONS, AND A SUBSTANTIALLY UNIFORM ROATION PREDOMINANTLY DEVOIDE OF VELOCITY GRADIENTS AND ANGULAR ACCELERATION TO PRODUCE AN EQUIAXED SOLIDIFICATION STURCUTRE IN THAT PORTIO OF THE OBJECT SOLIDIFIED DURING SAID UNIFORM ROTATION.
 2. The method as in claim 1, in which said molten metal preparation is comprised of aluminum.
 3. The method as in claim 2, in which said inoculant is comprised of Al2 O3.
 4. The method as in claim 2, in which said inoculant is comprised of at least one of boron and titanium.
 5. The method as in claim 4, in which said inoculant is added in amounts such that the final metallic object contains between 0.002-0.003 percent of either titanium or boron.
 6. The method as in claim 1, in which said molten metal preparation comprises lead.
 7. The method as in claim 2, in which the inoculant is selected from the group consisting of silver oxide, aluminum oxide, boron and titanium.
 8. The method as in claim 6, in which the inoculant is added in an amount sufficient to constitute no greater than 0.30 copper percentage by weight in the metal object.
 9. The method as in claim 1, in which the molten metal preparation comprises magnesium.
 10. The method as in claim 9, in which the inoculant comprises zirconium.
 11. The method as in claim 10, in which zirconium is added in an amount sufficient to constitute no greater than 0.65 percent by weight of the final metal object.
 12. The method as in claim 9, in which said inoculant is selected from the group consisting of aluminum, zirconium, calcium, zinc, silicon and rare earth metals.
 13. The method as in claim 6, in which said inoculant is selected from the group consisting of copper and calcium.
 14. The process of controlling the grain structure of a solidifying metal object comprising adding to the molten metal an inoculant which would normally produce a fine grained structure in the metal object on solidification, subjecting the metal during solidification to a plurality of mild angular accelerations during the time solidification is taking place in that portion of the object in which a columnar structure is desired, and subjecting the metal to centrifugal force generated by slow rotation during the time solidification is taking place in that portion of the object in which an equiaxed structure is desired.
 15. The process recited in claim 14 in which the metal object is essentially an object of revolution and the angular accelerations and slow rotation have an axis which is essentially the axis of revolution of the object.
 16. The process recited in claim 14 in which the solidifying metal object is subjected to a plurality of mild angular accelerations immediately after casting so that the outer layer of metal is columnar in structure. 